With the advantages of low cost, light weight, low operating voltage, high brightness, robustness, color tunability, wide viewing angle, ease of fabrication onto flexible substrates as well as low energy consumption, OLEDs are considered to be remarkably attractive candidates for flat panel display technologies and for solid-state lighting. Phosphorescent heavy metal complexes are an important class of materials in making OLEDs because of their relatively long triplet excited-state luminescence lifetimes and high luminescence quantum yields. The presence of a heavy metal center can effectively lead to a strong spin-orbit coupling and thus promotes an efficient intersystem crossing from its singlet excited state, eventually to the lowest-energy triplet excited state followed by relaxation to the ground state via phosphorescence at room temperature. This results in a four-fold enhancement on the internal quantum efficiency of the OLEDs up to 100%.
Typically an OLED consists of several layers of semiconductor sandwiched between two electrodes. The cathode is composed of a low work function metal or metal alloy deposited by vacuum evaporation, whereas the anode is a transparent conductor such as indium-tin oxide (ITO). Upon the application of a DC voltage, holes injected by the ITO anode and electrons injected by the metal cathode will recombine to form excitons. Subsequent relaxation of excitons will then result in the generation of electroluminescence.
The breakthroughs that led to the exponential growth of this field and to its first commercialized products can be traced to two pioneering demonstrations. In 1987, Tang and VanSlyke [Tang, C. W.; VanSlyke, S. A. Appl. Phys. Lett. 51, 913 (1987)] proposed the use of a double-layer structure of vacuum deposited, small-molecular films, in which tris(8-hydroxyquinoline)aluminum(Alq3) was utilized both as light emitting layer and electron transporting layer. Later, the first polymeric light emitting device was pioneered by Burroughs et al. in 1990 [Burroughs, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, N.; Friend, R. H.; Burn, P. L.; Holmes, A. B. Nature 347, 539 (1990)], in which a yellow-green electroluminescence from poly(p-phenylenenvinylene) (PPV) was achieved. Since then, a number of new electroluminescent small molecular based and polymeric light emitting materials have been investigated with improved light emitting properties. The key advantage of using polymers as light emitting materials is that they are highly soluble in most organic solvents, and the devices can be easily fabricated by using low-cost and efficient wet processing techniques, such as spin-coating, screen-printing, or ink-jet printing [Burrows, P. E.; Forrest, S. R.; Thompson, M. E. Curr. Opin. Solid State Mat. Sci. 2, 236 (1997)].
Apart from the development of small molecular and polymeric materials, recent demonstrations of the design and synthesis of dendrimers as light emitting materials provide new and interesting observations. Dendrimers are branched macromolecules consisting of repetitive units (dendrons) having a well-defined size and number of peripheral groups. These materials are typically comprised of three parts: a core unit, surrounding dendrons, and peripheral groups. The branching levels of the surrounding dendrons determine the dendrimer generation, in which the peripheral groups attached onto the surface of the surrounding dendrons can control the intermolecular interactions, solubility, viscosity and processability of the dendrimers. The emissive chromophores of the dendrimers can be located at the core of the dendrimer, within the surrounding dendrons or at the peripheral groups of the dendrimers. Typically, emissive chromophores are attached at the core units. In general, dendrimers can be divided into two classes, conjugated dendrons and saturated dendrons. The branching point of the conjugated dendrons or dendrimers must be fully conjugated but not essentially delocalized [Burn, P. L.; Lo, S. C.; Samuel, I. D. W. Adv. Mater 19, 1675 (2007)].
The distinct properties of dendrimers allow them to be good candidates in making OLEDs. Unlike polymers, dendrimers show a well-defined structure and precise molecular weight, in which the purity of the products can be well controlled and are reproducible; both of which are crucial factors for commercialization. In addition, their high solubility in most organic solvents opens up a possibility to fabricate the devices by solution-processed techniques such as spin-coating and ink jet printing. Such techniques not only are essential for the patterning of large-area displays and solid-state lighting panels, but also avoid the use of the expensive high-temperature vacuum evaporation techniques that are needed for preparing small molecular based OLEDs. More importantly, the dendrimer generation can control the intermolecular interactions. Intermolecular interactions are well-known to have an influence on the efficiency of OLEDs. Indeed, many emitters show strong luminescent properties in solution. However, the strong intermolecular interactions present in the solid state lead to the formation of dimers, excimers or aggregates that lower the efficiency of the OLEDs. At high current density, triplet-triplet annihilation tends to further lower device performance. In view of these properties, the introduction of bulky peripheral groups can keep the molecules apart and thereby avoid these problems. Furthermore, the color of light emission can be fine-tuned by simply choosing different combinations of the cores, dendrimers, and type of peripheral groups. For instance, compounds with the same branching level of surrounding dendrons and surface groups attached to different cores can lead to different color emission. The glass transition temperature of these macromolecules is usually high, giving a good operation stability to the devices [Liu, D.; Li, J. Y. J. Mater. Chem. 19, 7584 (2009)].
The first OLED with dendrimers as light emitting materials was demonstrated by Wang et al. [Wang, P. W.; Liu, Y. J.; Devadoss, C.; Bharathi, P.; Moore, J. S. Adv. Mater 8, 237 (1996)]. These dendrimers contained a highly fluorescent core, 9,10-bis(phenylethynyl)-anthracene, phenylacetylene as surrounding dendrons for electron capture, and tertiary butyl groups as the peripheral groups that maintained its solubility. Such devices exhibited two major photoluminescence bands at 480 and 510 nm, and a broad, structureless emission band at 600 nm; however, the device performance was rather low and indeed no efficiency data was reported. Later, Halim et al. reported a family of conjugated light emitting dendrimers based on PPV structure [Halim, M.; Pillow, N. G; Samuel, I. D. W.; Burn, P. L. Adv. Mater 11, 371 (1999)]. These dendrimers consisted of a distyrylbenzene core for blue color emission, stilbene dendrons, and t-butyl peripheral groups for solution processing properties. All three generations of dendrimers could be spin-coated from a chloroform solution to form amorphous thin films that produced a blue emission in the photoluminescence spectrum. A red shift was observed in the electroluminescence spectrum for the first generation dendrimer. With increasingly bulky groups to form different generations of dendrimers, concentration quenching effects were substantially suppressed. This demonstrated that dendrimers can effectively prevent the intermolecular interactions and the formation of dimers, excimers or aggregates.
While solution-processable fluorescent OLEDs have been realized, their efficiencies are usually quite low, and can be as low as 0.1%. In order to improve the device performance, it is desirable to make use of spin-orbit coupling in order to mix both singlet and triplet excited states. Hence, the use of heavy metal complexes in OLEDs is preferred over purely organic materials. Recently, a series of green-emitting carbazole conjugated dendrimers containing iridium(III) complexes have been reported by Ding et al. [Ding, J. Q.; Gao, J.; Cheng, Y.; Xie, Z; Wang, L. X.; Ma, D.; Jing, X. B.; Wang, F. S. Adv. Funct. Mater. 16, 571 (2006)]. By taking advantage of the dendritic structure(s), high solubility, non-doped, low cost solution-processable OLEDs had been achieved. By increasing the size of the dendrons, the intermolecular interactions can be significantly reduced, and good hole-transporting properties of carbazoles can be obtained. Superior device performance including peak external quantum efficiency (EQE) of 10.3% and current efficiency (CE) of 34.7 cd A−1 were achieved for a non-doped green OLED. Red-emitting triphenylamine dendrimers containing iridium(III) complexes were also reported in 2007 [Zhou, G. J.; Wang, W. Y.; Yao, B.; Xie, Z. Y.; Wang, L. X. Angew. Chem. Int. Ed. 46, 1149 (2007).] The extended π-conjugated system of triphenylamine dendrons raises the highest occupied molecular orbital (HOMO) level, and the electron-rich triphenylamine moieties facilitate an efficient hole injection from the anode. High EQE and CE of the devices of 7.4% and 3.7 cd A−1, respectively, were reached, even higher or comparable to the vacuum deposited devices with similar Commission Internationale de L'Eclariage (CIE) color. This indicates that dendrimers are one class of the promising light emitting materials for solution-processable OLEDs.
Even though there has been an increased interest in electrophosphorescent materials, particularly metal complexes with heavy metal centers, most of the development work has been focused on the use of iridium(III), platinum(II) and ruthenium(II), whereas the use of other metal centers has been much less explored. In contrast to the isoelectronic platinum(II) compounds which are known to exhibit rich luminescence properties, very few examples of luminescent gold(III) complexes have been reported, probably due to the presence of low-energy d-d ligand field (LF) states and the electrophilicity of the gold(III) metal center. One way to enhance the luminescence of gold(III) complexes is through the introduction of strong σ-donating ligands, which was first demonstrated by Yam et al. in which stable gold(III) aryl compounds were synthesized and found to display interesting photoluminescence properties even at room temperature [Yam, V. W. W.; Choi, S. W. K.; Lai, T. F.; Lee, W. K. J. Chem. Soc., Dalton Trans. 1001 (1993)]. Afterward, Yam et al. synthesized a series of bis-cyclometalated alkynylgold(III) compounds using various strong σ-donating alkynyl ligands, and all these compounds were found to exhibit rich luminescence behaviors at both room and low temperatures in various media [Yam, V. W.-W.; Wong, K. M.-C.; Hung, L.-L.; Zhu, N. Angew. Chem. Int. Ed. 44, 3107 (2005); Wong, K. M.-C.; Hung, L.-L.; Lam, W. H.; Zhu, N.; Yam, V. W.-W. J. Am. Chem. Soc. 129, 4350 (2007); Wong, K. M.-C.; Zhu, X.; Hung, L.-L.; Zhu, N.; Yam, V. W.-W.; Kwok, H. S. Chem. Commun. 2906 (2005)]. Very recently, a new class of phosphorescent material of cyclometalated alkynylgold(III) complexes has been reported and fabricated by vapor deposition [Au, V. K.-M.; Wong, K. M.-C.; Tsang, D. P.-K.; Chan, M. Y.; Yam, V. W.-W. J. Am. Chem. Soc. 132, 14273 (2010)]. The optimized OLED reached a EQE of 11.5% and CE of 37.4 cd A−1. This suggested that the alkynylgold(III) complexes are promising phosphorescent materials in terms of efficiency and thermal stability.
The present invention discloses herein the design, synthesis and photoluminescence behaviour of luminescent gold(III) dendrimers, and their device fabrication using solution-processing techniques to produce high efficiency dendrimer OLEDs. These devices combine saturated and conjugated dendrimers, containing at least one strong σ-donating group and a cyclometalated tridentate gold (III) compound. These novel devices can be fabricated using low cost, high efficiency, solution-processing techniques to obtain phosphorescence-based OLEDs that do not exhibit the limitations of known OLEDs.